1. Field of the Invention
The present invention relates to a method and device for measuring a flow of positive ions from an ionized gas, or plasma, to a solid surface in contact therewith, for example, a wall of a plasma reactor or a sample to be processed. The present invention more specifically applies to measuring an ion flow in an enclosure constituting a plasma reactor for coating a sample with a thin layer, or modifying the structure or the chemical composition of a surface by ion bombardment.
2. Discussion of the Related Art
FIG. 1 schematically shows, in a cross-sectional view, an example of a plasma reactor to which the present invention applies. It can be, for example, a so-called capacitive coupling radiofrequency excitation reactor.
Such a reactor is made of a vacuum enclosure 1. Close to a first wall 2 of this enclosure 1 is placed, on a wafer support 3, a sample 4 to be processed. Sample 4 is generally shaped as a disk having a surface 8 directed towards the inside of enclosure 1 which constitutes the surface to be processed. Enclosure 1 is filled with a low pressure gas, for example, of around a few tens to a few hundreds of millitorrs (a few tens of pascals). Several means can be used to generate the plasma. For example, in a so-called "capacitive coupling reactive ionic etching" configuration, a radiofrequency voltage is applied to the wafer support. As shown in FIG. 1, the plasma can also be generated by means of a source 6 independent from wafer support 3. This source 6 is, for example, for a d.c. voltage discharge, an electrode independent from the wafer support and supplied by a radiofrequency generator, an inductive coupling radiofrequency source (often associated with the application of a magnetic field) or a microwave source (which may be associated with the application of a magnetic field). In the case of the use of a source 6 independent from wafer support 3, the latter can be biased by a radiofrequency source 5 (FIG. 1) to establish a self-biasing and thus increase the ion impact power on the surface to be processed.
In a plasma etching or deposition method, it is important to know the characteristics of the interaction between the surface to be processed and the plasma to be able to control the implementation of the method, especially, to control the deposition or etching rate, according to the desired thickness of the deposition or depth of the etching. The flow of charged particles (ions and electrons) which arrive and leave the surface to be processed enables to determine these characteristics which depend, notably, on the plasma used.
A so-called induced fluorescence method enables to determine, in certain limited cases, the ion speed distribution function. However, such a method does not allow to definitely determine the ion flow. Moreover, its implementation is particularly complex and very costly.
The present invention applies to a direct electrical measurement of the ion flow in a plasma reactor.
Several methods are conventionally used to determine the characteristics of plasma reactors based on electrical measurements.
A first, so-called "Langmuir probe", method consists in inserting, in the middle of the plasma and thus away from the enclosure walls, a small generally cylinder-shaped electrode. This electrode is connected, outside the enclosure, by a wire surrounded with an insulating sheath. A variable voltage V is applied between the probe and the walls of the reactor and the current I in the wire is measured. The shape of the current-voltage characteristic I(V) thus obtained enables to estimate parameters characteristic of the plasma, such as the ion and electron density, the electron temperature or the plasma potential. With a modeling, these parameters enable to obtain an estimate of the ion flow towards the walls.
A so-called "planar Langmuir probe" alternative of this method consists in placing, next to a wall (for example, wall 9 in FIG. 1) of enclosure 1, an electrode shaped as a disk having a relatively large surface S (for example, a few square cm) with its rear surface directed to the wall coated with an insulating material.
FIG. 2 shows the shape of the current-voltage characteristic of such an electrode in a plasma reactor. When a strongly negative voltage V is applied, a saturation current Isat is reached. This current Isat is an image of the flow of positive ions .GAMMA..sub.ion since all the electrons are repelled. The relationship which links current Isat to ion flow .GAMMA..sub.ion, assuming that all the ions are ionized only once, is given by relation Isat=e.S..GAMMA..sub.ion, where e stands for the charge of an electron.
A disadvantage of Langmuir probe methods, which consist in measuring a d.c. current between the probe and the plasma, is that they no longer operate when the probe is contaminated, in particular if the plasma deposits an insulating layer on the electrode. This generally occurs with chemically complex gases (CF.sub.4, SiH.sub.4, CH.sub.4, etc.) which quickly deposit thin insulating layers on any surface in contact with the plasma.
A second method consists in sampling the ion (and electron) flow by means of a small aperture (generally having a diameter of approximately 100 .mu.m) in an electrode placed in the vicinity of an enclosure wall. An electrostatic filter placed behind the aperture enables one to separate positive ions from electrons and thus to measure the transmitted ion current. A disadvantage of such a method is that it requires a calibration of the transmission rate of the aperture and of the electrostatic filter. Yet, depositions of thin layers on the filter result in an alteration of the rate. The measurements are thus disturbed by these plasma-induced depositions, which makes them quickly unexploitable and results in complete failure of the measurement device.
A consequence of the disadvantages of the methods described hereabove is that conventional plasma reactors are generally characterized by operating with a rare gas, for example argon, previously to any deposition or etching method. The characteristics of a reactor in the presence of a complex gas thus cannot be known otherwise than by modeling.
Another disadvantage common to all known methods is that they do not allow any direct measurement of the ion flow during the processing of a sample. They thus do not allow any control of a deposition or etching method.